1. Field of the Invention
The present invention relates to a dynamic focus circuit generating a dynamic focus voltage, suitable for use in a wide-angled cathode ray tube (referred to as a CRT hereafter).
2. Description of the Related Art
In a CRT, a direct current (DC) voltage is typically applied as a focus voltage to a focus electrode in an electron gun. This DC voltage is produced by dividing the anode voltage to give a voltage with one quarter to one third times the magnitude. However, the distance the CRT electron beam travels to reach a screen varies between the center and the edges of the screen, so that obtaining satisfactory focus across the whole screen by applying only the DC voltage to the focus electrode is impossible. The amount of voltage applied to the focus electrode needs to be determined according to the distance between the focus electrode and a phosphor surface of the screen, that is according to the screen position at which the electron beam is focused. In a conventional CRT, voltage variations are commonly expressed by parabolic waveforms having horizontal and vertical periods. These waveforms are hereafter referred to as the horizontal and vertical parabolic waveforms. The voltage expressed by these parabolic waveforms is combined with the DC voltage and the resulting voltage is applied to the focus electrode. A circuit for generating modified horizontal and vertical parabolic waveforms so that the appropriate focus voltage can be applied to the focus electrode is known as a dynamic focus circuit.
One example of a dynamic focus circuit in the related art is given in Japanese Laid-Open Patent 4-117772 and Television Gakkai Gijyutsu Hokoku (The Television Society's Technical Report) Vol 17, No. 71, P 19 to 24 (published Nov. 18, 1993). The dynamic focus circuit disclosed here uses a method in which the voltage of a parabolic waveform signal generated at both ends of an S-shaping capacitor in a horizontal deflection circuit is raised directly using a step-up transformer. This method is hereafter referred to as the `S-shaping voltage method`. The following is an explanation of a related art example, a dynamic focus circuit using this S-shaping voltage method, with reference to the drawings.
FIG. 1 shows an example of a horizontal deflection circuit, and a related art dynamic focus circuit, which have been integrated to form one circuit.
The horizontal deflection circuit shown in the drawing includes a horizontal output transistor 901, into the base of which a horizontal drive signal is input, a damper diode 902, a resonance capacitor 903, a choke coil 904, a horizontal deflection coil 905, a S-shaping capacitor 906, and an alternating current (AC) coupling capacitor 909. The dynamic focus circuit is structured so that a dynamic focus voltage can be obtained by raising the voltage of a parabolic waveform signal generated at both ends of the S-shaping capacitor 906 using a step-up transformer 908.
The horizontal output transistor 901, the damper diode 902 and the resonance capacitor 903 are connected in parallel, and the collector side of the horizontal output transistor 901 is connected to a +B power source through the choke coil 904. The collector side of the horizontal output transistor 901 is also connected to one terminal of the horizontal deflection coil 905. The other terminal of the horizontal deflection coil 905 is connected to the S-shaping capacitor 906.
One terminal of the primary coil of the step-up transformer 908 is connected to a node 907, where the horizontal deflection coil 905 and the upper end of the S-shaping capacitor 906 connect. The AC coupling capacitor 909 is connected to the other terminal of the primary coil of the step-up transformer 908.
In the dynamic focus circuit, a vertical dynamic focus voltage waveform generating circuit 912 (hereafter referred to as the vertical voltage waveform generating circuit 912) and a capacitor 914 are connected to one terminal of the secondary coil of the step-up transformer 908. The other terminal is coupled to a DC focus voltage generating circuit 913 through a resistor 910 and an AC coupling capacitor 911. The dynamic focus circuit is structured so as to be connected to the focus electrode in the electron gun.
FIG. 2 shows waveforms produced in various parts of the above dynamic focus circuit. Horizontal collector pulses 921 are generated at the collector side of the horizontal output transistor 901 by the resonance of the horizontal deflection coil 905, the choke coil 904 and the resonance capacitor 903. A secondary integration operation of the horizontal deflection coil 905 and the S-shaping capacitor 906 generates a horizontal parabolic voltage 922 in the S-shaping capacitor 906. When the horizontal parabolic voltage 922 is applied to the primary coil of the step-up transformer 908, a dynamic focus voltage 923 for a horizontal deflection period is output from the secondary coil of the step-up transformer 908.
A vertical dynamic focus voltage generated by the vertical voltage waveform generating circuit 912 is added to the horizontal dynamic focus voltage by being input into the other terminal of the secondary coil of the step-up transformer 908. The dynamic focus voltage obtained is passed through the resistor 910 and the AC coupling capacitor 911 and combined with a DC voltage obtained from the DC focus voltage generating circuit 913. The resulting voltage is then supplied to the focus electrode in the electron gun, enabling an ideal focus to be obtained across the entire screen.
As explained above, the related art dynamic focus circuit uses the S-shaping voltage method to obtain a dynamic focus voltage waveform by raising the voltage of a parabolic waveform signal generated at both ends of an S-shaping capacitor in a horizontal deflection circuit using a step-up transformer.
In recent years, however, CRTs with a wider deflection angle and less depth (hereafter referred to as wide-angled CRTs) are increasingly being used to enable space-saving display devices with large screens to be produced. When compared with a conventional device, a wide-angled CRT experiences a sharp increase in distortion between the plane at which the electron beam is focused and the surface of the phosphor layer as the electron beam moves towards the edges of the screen, if a uniform focus voltage is used. Accordingly, when a related art parabolic waveform proportional to the square of the distance from the center of the screen is used as the horizontal focus voltage waveform, obtaining satisfactory focus across the entire screen is problematic.
The results of our investigation into the focus characteristics of a wide-angled CRT, in light of the above problems, are shown in FIG. 3. In the drawing, a dashed line (curve a) represents a quadratic curve of a dynamic focus voltage in the related art. A solid line (curve b) represents the dynamic focus voltage suitable for a wide-angled CRT. Curve a is proportional to the square of the distance from the center of the screen. Curve b is obtained by making a mirror image of a curve proportional to the distance from the center of the screen raised to the power of around 2.5 for the right half of the screen. This produces a curve in which the left and right halves are symmetrical. In a wide-angled CRT, the distortion between the plane at which the electron beam is focused and the surface of the phosphor layer increases sharply as the electron beam moves from the center to the edges of the screen, as shown here. As a result, a dynamic focus voltage having a waveform that is flatter than the related art in the center of the screen, and rises more steeply towards the edges of the screen (this waveform is hereafter referred to as `the flat-bottomed waveform`) is required to obtain satisfactory focus characteristics across the entire screen. As shown in FIG. 3, there is a large voltage difference between the two curves at either edge of the screen and in an area from 60 to 140 mm on either side of the center of the screen, so that applying a dynamic focus voltage with a conventional parabolic waveform in a wide-angled CRT leads to a deterioration in focus in these areas of the screen.
One possible approach to resolving this problem is suggested in Japanese Laid Open Patent No. 10-42162. Here, this document describes a dynamic focus circuit shown in FIG. 4, including a ROM 931 for storing function data, a counter 932 initialized by a synchronizing signal S, a RAM 933 for storing waveform data, and a CPU 934 for performing calculations. The CPU 934 uses function data already stored in the ROM 931 to perform computation of waveform data depending on screen positions, and stores the results in the RAM 933. Next, the waveform data is read from the RAM 933 and output to a D/A converter 935, which converts it from digital to analog data, and outputs a dynamic focus voltage waveform. In other words, in the above related art dynamic focus circuit, digital processing is used to generate a waveform expressed by a complex function.
Digitalization of circuitry in display devices using CRTs has become more common in recent years, but a method for generating a digitalized dynamic focus signal cannot easily be introduced due to cost constraints, and so analog circuits are still generally preferred. Even the dynamic focus circuits used in high-definition computer monitors are mainly analog.